Switching voltage regulators generate an output voltage from an input voltage and are implemented with active components such as a pulse width modulation controller (PWM), driver, power MOSFETs, and passive components such as inductors, transformers or coupled inductors, capacitors, and resistors. Controllers typically measure the output current and output voltage in order to regulate and monitor the output voltage. Measuring the input current and input voltage allow the controller to improve regulation and monitoring, as well as monitoring input power for voltage, current, and power constraints.
Knowledge of the regulator input current and voltage is required in some controllers for improved regulation algorithms and monitoring input power for voltage, current, and power constraints. Measuring the input current and voltage requires external and internal circuitry in the controller. For example, some regulator controllers differentially sense a small signal biased with a high DC common mode voltage such as measuring the DCR drop across an input inductor. Matched resistors are typically used in this case to level shift the high bias to voltages at a level the controller can tolerate. However, due to the large drop in voltage, any mismatch in resistance between the level shifting resistors can cause offset issues. Also, due to the small input signal levels, a high gain input stage is needed.
However, this limits the amount of offset the input stage can tolerate before the ADC (analog-to-digital conversion) conversion range of the input stage saturates. In a typical application of measuring a 2 milliohm input current using a sense resistor biased at 12V, resistance tolerance better than 0.1% is required. Also, prior to the resistors being put down on the system board, the sign and magnitude of the mismatch cannot be determined. This means the system must be designed such that enough ADC range is provided to allow for resistor mismatch in both polarities. In addition, high precision resistors are very expensive. For example, resistors with 0.1% precision cost more than 10 times resistors with 1% precision.
Hence, there is a need for an improved input current and voltage monitoring technique for switching voltage regulators which minimizes regulator complexity and cost.